Reddit mentions of Fish Forever: The Definitive Guide to Understanding, Selecting, and Preparing Healthy, Delicious, and Environmentally Sustainable Seafood

The no-sushi-while-pregnant thing is mostly a myth. It drives me absolutely crazy -- expectant moms have enough to worry about already without having that particular joy snatched from them. I assure you, japanese women don't stop eating sushi when they're expecting.

There are two major concerns that cause paranoid doctors (and an even more paranoid mom-blaming media) to advise against sushi: mercury and parasites. With a little research you can figure out which fish to avoid.

Bluefin tuna would tend to have high mercury content. Fish that live a really long time and are near the top of the food chain are the ones to worry about.

Freshwater fish would tend to be more of a parasite risk, but a good sushi place would generally be careful to source clean fish. Generally, raw or cold-smoked salmon should probably be avoided.

Oh, and shellfish pose relatively more of a food-poisoning risk than other foods because you're eating all of their guts.

You're probably fine to eat short-lived ocean fish like mackerel. I believe yellowtail's pretty safe too. Unagi's cooked anyway, so go crazy. Scallops are quite safe because you're only eating the muscle (unlike other shellfish).

This is a really great resource that will help you balance concerns around mercury and getting adequate omega-3: http://www.amazon.ca/Fish-Forever-Understanding-Environmentally-Sustainable/dp/076458779X

(Everything I'm saying here is off the top of my head, so definitely double-check and don't bet your or your baby's health on my internet ranting!)